1. Field of the Invention
The present invention relates to a process for preparing barium titanate (BaTiO.sub.3) single crystals which exhibit high photorefractive properties and can be applied to optical-devices such as phase-conjugate mirrors, laser resonators, optical image-processing, etc.
2. Description of Related Art
Known methods of preparing single crystals of BaTiO.sub.3 include the flux method in which potassium fluoride (KF), barium chloride (BaCl.sub.2) or the like is used as a flux material (J. P. Remeika et al., J. Am. chem Soc., vol. No.76, p.p. 940-41, (1954)). The single crystal of BaTiO.sub.3 prepared in this method is called a butterfly type crystal, which will have triangle shapes of about 1 mm in maximum thickness. With this method, it is impossible to produce single crystals of BaTiO.sub.3 which are large enough and thick enough to be applied to optical devices.
After the flux method, the Top Seeded Solution Growth technique (T.S.S.G technique) for preparing single crystals of BaTiO.sub.3 was developed. In this method, the crystals of BaTiO.sub.3 are grown on seed crystal (BaTiO.sub.3 single crystal), while a melt containing titanium dioxide (TiO.sub.2) in excess of BaTiO.sub.3 composition is gradually cooled.
This method allows the single crystals of BaTiO.sub.3 to form a desired bulk shape, and to be less contaminated by the flux and other impurities. Therefore, the single crystals of BaTiO.sub.3 prepared by this method are excellent in optical characteristics compared to those prepared by the flux method. For this reason, studies for applying BaTiO.sub.3 single crystals to optical devices as photorefractive crystals have developed actively (see. Kitayama, Japan Society of Applied Physics, Crystal Engineering Section, the text for the 95th Study Meeting, p.p. 13-17 (1991)).
Recently, a more improved form of BaTiO.sub.3 single crystal in its photorefractive properties has been demanded by the technologies which need to use photorefractive crystals of this kind. Furthermore, it is now believed that the photorefractive properties of the BaTiO.sub.3 single crystal is dependent upon crystal defects such as oxygen vacancies or the presence of impurities e.g. transition metal elements etc. in the single crystal.
For this reason, in order to enhance the photorefractive properties of BaTiO.sub.3 single crystals the method has ben proposed in which single crystals of BaTiO.sub.3 grown in the air are heated in an atmosphere having a low partial pressure of oxygen or in a reducing atmosphere (P. G. Schunemann et al, J. Opt. Soc. Am. B., Vol. No.5, p.p. 1702-10 (1988)). Another method for enhancing the photorefractive properties has been proposed in which transition metal elements such as Fe and Cr, etc. are doped into single crystals of BaTiO.sub.3 (D. Rytz et al., J. Opt. Soc. Am. B., Vol. No.7, p.p. 2245-54, (1990)). Furthermore, another method has been disclosed in which a mixture of substances, including BaTiO.sub.3 single crystals, is melted, and thereafter the mixture is placed in a CO/CO.sub.2 atmosphere in order to grow a single crystal of BaTiO.sub.3 (U.S. Pat. No. 4,724,038).
It has also been found that oxygen vacancies in the BaTiO.sub.3 single crystal grown in air can be increased much more by heating in an atmosphere having a lower partial pressure of oxygen or a reducing atmosphere. It is also possible to increase the content of transition metal elements in the single crystal by doping impurities such as Fe, Cr and the like into the BaTiO.sub.3 single crystal, to thereby enhance the photorefractive properties. Furthermore, a single crystal of BaTiO.sub.3 which has an increased number of oxygen vacancies can be obtained by growing the single crystal in a reducing atmosphere containing CO gas. Thus the single crystals of BaTiO.sub.3 prepared in these methods exhibit relatively great photorefractive properties.
The photorefractive properties of the BaTiO.sub.3 single crystals prepared by the methods reported up to date are, however, still insufficient for practical use. There are still many uncertain factors in the relationship between the photorefractive properties of BaTiO.sub.3 single crystals produced and the effects of the amount of doping transition metal elements or the amount of heating in a reducing atmosphere mentioned above. In conclusion, it is difficult based on what is conventionally known to stably prepare or provide BaTiO.sub.3 crystals exhibiting good photorefractive properties.